In the petroleum and petrochemical industries, many hydrocarbon gas streams, such as but not limited to, steam cracker gas, fluidized catalytic cracking (FCC) gas and refinery fuel gas, contain acid gases (compounds such as CO2, H2S and mercaptans) that require treating for acid gas removal for a variety of reasons. For example, acid gas removal may be required for meeting product specifications, preventing poisoning of downstream catalyst beds, odor control and/or upgrading fuel calorific value.
Bulk acid gas removal is achieved in amine towers and caustic towers are then used to achieve acid compound removals down to low parts per million concentration levels. The removal of CO2 and H2S from hydrocarbon gas streams is known to be achieved by intimate contact with an aqueous solution of a base, such as sodium hydroxide (NaOH), which is a caustic solution. By reaction with the caustic of the caustic solution, i.e., NaOH, acid gas components, such as hydrogen sulfide and carbon dioxide are converted into sodium sulfide (Na2S), sodium hydrosulfide (NaHS), sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3), which are absorbed into the caustic solution and, thus, removed from the hydrocarbon gas stream.
One type of petrochemical operation wherein an aqueous sodium hydroxide solution almost invariably is used for gas scrubbing is in an ethylene production unit or plant (although others are contemplated by the present invention, a description of the ethylene production unit will be made for simplicity sake). In an ethylene plant, a saturated aliphatic hydrocarbon feed, such as ethane, propane or higher oils, and the like, is heated at high temperatures in the presence of steam to crack the saturated hydrocarbon molecules down to lower molecular weight unsaturated hydrocarbons, such as predominately ethylene, followed by propylene and then various quantities of C4, C5 and C6 mono- and di-olefinic hydrocarbons, with lesser quantities of C7 and higher weight saturated and unsaturated aliphatic, cyclic and aromatic hydrocarbons.
During steam cracking, any sulfur containing compounds added to and/or present in the hydrocarbon feed stream are converted into hydrogen sulfide and/or organically bound sulfur compounds and, also a content of carbon dioxide is generated by a water-gas shift reaction. The resultant gas mixture from steam cracking then is quenched from a temperature ranging from about 700 to about 1000° C. to a lower temperature ranging from about 35 to about 40° C., whereupon the major portion of its water and C7+ hydrocarbon content is condensed and separated from the mixture. After quenching, the remaining constituents of the gas mixture conventionally are conditioned by various steps of gas compression and refrigerative cooling to prepare it for cryogenic distillation whereby its ethylene, propylene and butenes content ultimately will be recovered in essentially pure form for ultimate use as monomers in the production of various polymers, such as polyethylene, ethylene copolymers, polypropylene and the like.
One step required to properly condition the gas mixture for cryogenic distillation is to scrub (or otherwise clean) the cracked gas essentially free of any acid components, such as hydrogen sulfide and carbon dioxide. Conventionally, this has been accomplished at some inter-stage location of a multi-stage gas compression system and, on occasion post-compression, wherein the cracked gas stream is at a pressure ranging from about 10 to about 20 atmospheres (atm). The compressed gas stream is contacted with an aqueous sodium hydroxide solution by countercurrent contact in a gas-liquid contact vessel, often referred to in the industry as an “absorber,” “scrubber” or “caustic tower.” After such gas scrubbing, the aqueous sodium hydroxide solution, which is discharged from the bottom of this tower contains, in addition to some unreacted sodium hydroxide, the sodium sulfide, sodium hydrosulfide, sodium carbonate and sodium bicarbonate that results from the removal of acid gas compounds from the scrubbed gas stream.
To prevent build-up of the concentration of these components in the caustic tower and to provide for hydraulic room to add a quantity of fresh higher strength caustic solution to the caustic tower to make up for the consumption of caustic in the tower, a quantity of this weak or “tower spent” caustic solution is bled away from being recirculated back to the tower. To maintain a proper liquid volume of caustic solution circulation within the tower, however, a portion of this weak or “tower spent” caustic solution is recirculated back to the tower. That quantity of the weak or “tower spent” caustic solution bled away from the tower has been referred to in this art “spent caustic.” The spent caustic then is conditioned for environmentally sound disposal in a spent caustic treatment unit.
Some feedstocks to the steam cracker also contain substantial amounts of mercaptans, including relatively heavy mercaptans. These relatively heavy mercaptans largely will decompose to H2S and hydrocarbons and exit the furnace with the cracked gases, the undecomposed heavy mercaptans and lighter mercaptans. In the quench tower, the heavier mercaptans for the most part will condense and leave with the separated fuel oil, but the lighter mercaptans will end up with the cracked gas leaving the water quench tower for compression and further olefin purification. One of the steps for olefin purification conventionally used in olefin production facilities is acetylene hydrogenation, which typically employs a palladium catalyst. The mercaptans in the cracked gas stream must be substantially removed to prevent poisoning of the palladium catalyst.
The conventional methods for removing mercaptans and other sulfur compounds from the cracked gas stream have entailed the use of regenerable activated alumina adsorbent beds, with adsorbents such as Selexsorb COS, Selexsorb CD and Selexsorb CDX supplied by BASF, or by using non-regenerable catalysts, such as zinc oxide, copper oxide or lead oxide to form zinc sulfide, copper sulfide or lead sulfide, respectively. These commercially available methods, however, have proved very expensive.
Accordingly, it would represent a notable advance in the state of the art if a process for removing mercaptans could be developed, which is more economical than the processes taught in the prior art and which would not require the addition of another process step in addition to the caustic scrubbing.